1. Field of the Invention
This invention relates to a distortion compensator for correcting distortion occurring in an amplifier that amplifies transmit signals composed of different frequency carrier signals, an amplifier device equipped with the distortion compensator, a base station equipped with the amplifier device and a relay amplifier station equipped with the distortion compensator. This invention relates particularly to a technology that provides a distortion compensator appropriate for use with carrier signals having mutually different frequencies for improving accuracy of distortion compensation by reducing the effect of frequency characteristics.
2. Description of the Prior Art
In a mobile telecommunications system adopting wideband code-division multiple access (W-CDMA) technology, the base station (CDMA base station) is required to wirelessly transmit signals to reach CDMA mobile stations at remote physical locations. The transmit signals therefore need to be greatly amplified by an amplifier for output at high power.
Since an amplifier is an analog device, however, its input/output characteristics are nonlinear. The effect of this is particularly strong at and beyond the saturation point, known as the amplification limit, where the power of the signal output by the amplifier remains substantially constant irrespective of increase in the power of the input signal. This nonlinear output produces nonlinear distortion.
Therefore, even though signal components outside the desired signal band present in the signal are suppressed to a low level by a band limiting filter before amplification, nonlinear distortion occurs in the signal passed through the amplifier and causes leakage of signal components outside the desired signal band to adjacent channels, for example. The permissible level of such power leakage to adjacent channels is severely regulated in base stations because of their high transmission power. Reduction of adjacent channel leak power (ACP) has therefore become a topic of major interest.
One conventional technology for reducing adjacent channel leak power in a base station is the distortion-compensating transmit power amplification unit that uses a predistortion technique. The structure and operation of units of this type will now be explained.
FIGS. 13 to 16 show the configurations of various distortion-compensating transmit power amplification units utilizing predistortion. Each will be explained separately.
In the transmit power amplification unit of FIG. 13, the transmit signal to be amplified is input to a distortion compensator 41 that predistorts the signal to give it a distortion compensation characteristic. The signal is then amplified and output by a transmit power amplifier unit 42.
The distortion compensation characteristic can, for example, be a characteristic that is the inverse of the nonlinear characteristic occurring in the amplitude-phase plane when the signal is amplified by the amplifier unit 42. The inverse characteristic ordinarily used is that of AM (amplitude modulation)-AM conversion and AM-PM (phase modulation) conversion produced with input signal power as an index. So the distortion compensator 41 is imparted beforehand with distortion whose characteristic is the inverse of the nonlinear characteristic of the amplifier unit 42 with respect to the transmit signal input thereto. The distortion occurring when the transmit signal is amplified in the amplifier unit 42 is therefore corrected so that the signal output by the amplifier unit 42 is the input signal amplified without distortion.
The distortion compensator 41 is, for example, constituted using a diode or the like having the distortion compensation characteristic.
In the transmit power amplification unit shown in FIG. 14, the transmit signal to be amplified is input to a distortion compensator 51 and a power detector 53. The power detector 53 detects the power of the signal and outputs the detection result to a controller 54. The controller 54 controls the distortion compensator 51 to distort the signal to have a distortion compensation characteristic based on the power detection result. The signal imparted with distortion by the distortion compensator 51 in accordance with the control signal from the controller 54 is amplified by an amplifier unit 52.
When the signal input to the distortion compensator 51 is analog, the distortion compensator 51 is configured using an attenuator (ATT) for attenuating the signal and a phase shifter for shifting the signal phase. When the input signal is digital, the distortion compensator 51 is configured using a vector arithmetic unit for changing the amplitude and phase of the digital signal.
The power detector 53 can use any of various methods for detecting the signal power, including, for example, envelope detection or a method that uses a processor to conduct processing equivalent to envelope detection.
The controller 54 is can be configured using a digital signal processor (DSP) and is provided with a lookup table (LUT) regarding AM-AM conversion and AM-PM conversion for imparting distortion with a distortion compensation characteristic. When the signal input to the transmit power amplification unit of FIG. 14 is analog, the controller 54 supplies the distortion compensator 51 with information for controlling the attenuator and information for controlling the phase shifter as appropriate for the power of the input signal. When the input signal is digital, the controller 54 supplies the distortion compensator 51 with compensation vector information appropriate for the power of the input signal.
Further, when the signal input to transmit power amplification unit of FIG. 14 is analog, the controller 54 is equipped with an analog-to-digital (A/D) converter for digitizing the power detection result it receives as an analog signal from the power detector 53 and with an digital-to-analog (D/A) converter for converting control signals for the distortion compensator 51 from digital to analog and outputting them to the distortion compensator 51.
The transmit power amplification unit shown in FIG. 15 includes a distortion compensator 61, amplifier unit 62, power detector 63 and controller 65, which function like their counterparts in FIG. 14, plus a demodulator 64 that forms a feedback loop. In this transmit power amplification unit, the transmit signal input to the distortion compensator 61 and power detector 63 is also input to the controller 65, and part of the signal output by an amplifier unit 62 is input to the demodulator 64. The demodulator 64 orthogonally demodulates this portion of the signal and sends the orthogonal demodulation result to the controller 65.
Further, in this transmit power amplification unit, the controller 65 compares the transmit signal and the orthogonal demodulation result, detects the distortion component produced in the amplifier unit 62, and updates the lookup table so as to reduce the detected distortion component. The updating of the lookup table makes it possible, for example, to adapt to changes in the nonlinear characteristic of the amplifier unit 62 caused by aging, temperature change or the like. In other words, the transmit power amplification unit is capable of adaptive predistortion.
The transmit power amplification unit shown in FIG. 16 includes a distortion compensator 71, amplifier unit 72, power detector 73 and controller 75, which function like their counterparts in FIG. 14, plus a filter unit 74 that forms a feedback loop. In this transmit power amplification unit, part of the signal output by an amplifier unit 72 is input to the filter unit 74. The filter unit 74 extracts from this signal portion only the signal component in the band in which the distortion component produced in the amplifier unit 72 is leaked and sends the extracted distortion component to the controller 75.
Further, in this transmit power amplification unit, the controller 75 updates the lookup table so as to reduce the input distortion component thereto. The updating of the lookup table makes it possible, for example, to adapt to changes in the nonlinear characteristic of the amplifier unit 72 caused by aging, temperature change or the like. In other words, the transmit power amplification unit is capable of adaptive predistortion.
Then filter unit 74 can be constituted using a band pass filter (BPF) or mixer and variable frequency oscillator.
Prior art technologies related to distortion compensation of wideband signals will now be discussed.
Examined Japanese Patent Publication JP-B-7-101819 (Reference No. 1) teaches a “Distortion compensation circuit in a multifrequency simultaneous amplifier.” In this technology, when a composite signal obtained by synthesizing multiple different frequency RF input signals is amplified using a wideband low-distortion amplifier, a distortion component of the frequency of a calculated spurious component is detected from the amplified signal and a distortion component occurring in the composite signal is controlled to minimize the detected distortion component.
One difference between this technology and the present invention described in the following is that Reference No. 1 is silent regarding control of distortion compensation based on input signal level.
Unexamined Japanese Patent Publication JP-A-2000-223961 (Reference No. 2) teaches a “Feed-forward amplifier and amplification method.” In this technology, signals in different frequency bands are separately compensated for distortion by respective predistortion circuits, the distortion-compensated signals of all frequency bands are synthesized, the synthesized signal is amplified by a main amplifier, the distortion component produced by the main amplifier is detected by the feed-forward method, and the detected distortion component is removed from the amplified signal.
One difference between this technology and the present invention described in the following is that Reference No. 2 is silent regarding control of distortion compensation based on input signal level.
As is can be seen from the foregoing examples of the prior art, various distortion-compensating power amplifier configurations have been considered.
However, in the prior art distortion compensation configurations, such as illustrated in FIGS. 14 to 16, the distortion compensation characteristic, i.e., the AM-AM conversion and AM-PM conversion relative to the input signal power, is not uniform owing to the difference between the frequency characteristic present during the period the input signal passes through the distortion compensator and is output from the amplifier unit and frequency characteristic present in the power detector. The distortion compensation performance is therefore limited.
This disadvantage will be explained more specifically with reference to the graph of FIG. 17.
In FIG. 17, the curve (a) shows an example of the frequency characteristic of the amplitude of a signal input to the distortion compensator and the curve (b) shows an example of the frequency characteristic of the amplitude of the signal output from the amplifier unit (AMP) in this case. (The horizontal axis of the graph represents frequency and the vertical axis gain.)
As the graph shows, even if the frequency characteristic of input signal amplitude is linear, the frequency characteristic of the amplitude of the signal output by the amplifier unit is not linear but nonlinear. This nonlinearity arises owing to the frequency characteristics of the various components of the amplifier unit, such as the drive amplifier and the power amplifier unit. Owing to this nonlinearity, the power of the transmit signal input to the distortion compensator and the power of the signal input to the power amplifier that produces the distortion come to differ, making it impossible to carry out accurate distortion compensation.
Particularly in a base station adopting W-CDMA, the transmit signal amplified in the amplifier unit may contain multiple carrier signals differing in frequency. Since in such a case the frequency band of the signal to be amplified becomes still broader, the frequency characteristics appear strongly to make it impossible to eliminate their effect and perform accurate distortion compensation.
The graph of FIG. 17 illustrates only an example of amplitude frequency characteristic. The phase and other frequency characteristics are also nonlinear.
The “Feed-forward amplifier and amplification method” taught by Reference No. 2 can be considered to conduct distortion compensation using a separate predistortion circuit for the signal of each frequency band. Reference No. 2 is, however, silent regarding the point of how to control this kind of distortion compensation. If such distortion compensation is to actually operate effectively, consideration must be given to the means used to implement such control.
In other words, while Reference No. 2 can be understood to effect distortion compensation using multiple predistortion units for the respective signals of the frequency bands included in a wideband signal, it leaves untouched the matter of how the predistortion unit should be controlled. In contrast, the present invention, as explained below, focuses attention on this point. Specifically, the present invention reflects the thinking that simplification of the control configuration, ease of implementation of the control configuration, and improved control accuracy can better be achieved by control with consideration to the power level of a signal combining the signals of the multiple frequency bands, i.e., the signal to be amplified, than by control with consideration to the power level of the individual signals of frequency bands corresponding to the predistortion units.
The present invention was accomplished in light of the foregoing problems of the prior art and has as an object to provide a distortion compensator that compensates for distortion occurring in an amplifier that amplifies a signal while conducting control based on the level of the signal to be amplified and is characterized in that it improves distortion compensation performance by reducing the effect of frequency characteristics that restrict the distortion compensation performance limit.